The present invention relates to a double-cylinder construction in a compression molding machine for rubber, plastic, etc. In this specification and claims, said double-cylinder construction means a construction comprising an outer cylinder and a ram slidably fitted therein, said ram having a hollow part in the center thereof and serving also as an inner cylinder.
Double-cylinder constructions have been employed in large compression molding machines having a ram of a large diameter because the double-cylinder constructions have the advantage that they make it possible to use a hydraulic pump having a smaller capacity. FIG. 4 shows a typical conventional double-cylinder construction in a compression molding machine, which construction comprises an outer cylinder 1 and a ram or an inner cylinder 3 slidably fitted therein, said ram 3 being provided at the lower end thereof with a large-diameter portion 2, said ram 3 having a hollow part 4 in the center thereof, a rod 6 being fitted in said hollow part 4 so as to penetrate the bottom 5 of said outer cylinder 1, said rod 6 being slidable relative to said ram 3 and immovably fixed to the bottom of said outer cylinder 1, said rod 6 being provided in the center thereof with an oil passage 7 penetrating the rod 6 from end to end. A space formed by the lower surface of said large-diameter portion 2 of the ram 3, the inner surface of said outer cylinder 1 and the periphery of said rod 6 serves as a lower oil reservoir 8. A portion of said hollow part 4 of the ram 3 over the rod 6 serves as a ram raising oil reservoir 9. A space formed by the periphery of the ram 3 and the inner surface of the cylinder 1 serves as a ram lowering oil reservoir 10. Numeral 11 represents a check valve, and numeral 12 represents an oil tank.
In the double-cylinder construction described above, the ram 3 will be raised when an oil pressure is applied to the ram raising oil reservoir 9 through the oil passage 7 within the rod 6. The ram 3 will be lowered if an oil pressure is applied to the ram lowering oil reservoir 10 when the ram 3 is in a raised position.
In the conventional double-cylinder construction described above, the rod 6 is required to have a large diameter because the rod 6 is provided in the center thereof with the oil passage 7 penetrating the rod 6 from end to end. Furthermore, the rod 6 having a large diameter is required to penetrate the bottom 5 of the outer cylinder 1 in a state of being firmly fixed to said bottom 5. Therefore, the aforesaid conventional double-cylinder construction is used only in large-sized compression molding machines and not used in small-sized or medium-sized compression molding machines.
In the conventional double-cylinder construction described above, since all oil used for the operation thereof is supplied from outside, it is not necessarily possible to make external hydraulic apparatuses including the oil tank satisfactorily small and less expensive.